Process and apparatus for etching copper and copper alloys

ABSTRACT

Copper and copper alloys, in particular copper-coated laminates, are etched by immersing them in or spraying them with a solution of cupric chloride and regenerating the formed cuprous chloride during continuing operation by adding hydrogen peroxide and hydrochloric acid to the etching solution, the amount of the addition being controlled by measurements of the redox potential by means of a redox electrode and the control being adjusted to add predetermined amounts of the said two additives whenever the copper-I-ion concentration reaches a predetermined value, preferably 0.4 g/l so as to maintain a constant etching speed. An apparatus for use in this method comprises a tank for the etching solution, means for applying the solution to the copper or copper alloy material, a redox electrode immersed in the said tank, supply vessels for holding the regenerating additives, a conduit between said supply vessels and the tank, valve means for passing the regenerating liquid from the supply vessels to the tank and control means for operatively connecting the valve means to the redox electrode.

United States Patent [191 [451 Apr. 29, 1975 Rehm et a1.

[ PROCESS AND APPARATUS FOR ETCHING COPPER AND COPPER ALLOYS [73]Assignee: Hans Hoellmueller Maschinenbau,

Gueltstein, Germany 22 Filed: Nov. 5, 1969 21 Appl. No.: 874,100

[30] Foreign Application Priority Data Nov. 7, 1968 Germany 1807414 [52]US. Cl. 156/19; 23/230 A; 23/253 A; 156/345; 134/57 [51,] Int. Cl C23b3/00 [58] Field of Search.. 324/29, 30; 23/230 A, 253 A; 156/19, 345

[56] References Cited UNlTED STATES PATENTS 2,908,557 10/1959 Black156/19 2.927.871 3/1960 Mancke eta1.... 134/10 2.977.199 3/1961 Quittner23/230 A 3,000,385 9/1961 Shay 134/57 3,074,277 1/1963 Hill 134/573,117,954 1/1964 Hupfer 23/230 A 3,312,189 4/1967 McVcy 134/57 OTHERPUBLICATIONS Greer, Measurement and Automatic Control of the EtchingStrength of Ferric Chloride, pub. in Plating, pp. 1095 1096, Oct. 1961.

Primary Examiner-George F. Lesmes Assistant E.\'aminerR. .1. RocheAttorney, Agent, or FirmMichael S. Striker [57] ABSTRACT Copper andcopper alloys, in particular copper-coated laminates, are etched byimmersing them in or spraying them with a solution of cupric chlorideand regenerating the formed cuprous chloride during continuing operationby adding hydrogen peroxide and hydrochloric acid to the etchingsolution, the amount of the addition being controlled by measurements ofthe redox potential by means of a redox electrode and the control beingadjusted to add predetermined amounts of the said two additives wheneverthe copper-l-ion concentration reaches a predetermined value, preferably0.4 g/l so as to maintain a constant etching speed.

4 Claims, 4 Drawing Figures 15 k a a ll PATENTEDAPRZQIBYS 3.880.685

I SHEET 3 BF 4 sio V m/sec) sbo BACKGROUND OFTHE INVENTION The inventionconcerns a process for etching printed circuit boards in immersion andspray apparatus.

A variety of methods exists for etching conductor plates, e.g., printedcircuits. Known etching agents are aqueous solutions of ferric chloride(FeCl ammonium persulphate (NH S sodium chlorate and hydrochloric acid(NaClO HCl), sodium chlorite (NaClO hydrogen peroxide and sulfuric acid(H 0 H 80 and cupric chloride (CuCl with various regenerating agentssuch as air and oxygen. Immersion or spray apparatus is used foretching, the latter, in particular, in the form of a continuous processoperation particularly suitable for manufacture of conductive sheets ona conveyor belt. An economical conveyor belt-type manufacture isconditional on maximum and constant throughput speed. In most etchingprocesses so far known increasing copper content in the etching agentand decreasing etching agent power meant lengthened etching times. If,for example, the copper content rises from 0 to 50 g Cu/liter theetching time in an ammo nium persulphate bath rises from 6 to 33 minutesand in a hydrogen peroxide/sulfuric acid bath from to 21.5 minutes(German published application 1,253,008). In another process usinghydrogen peroxide and sulfuric acid plus a catalyst. etching times of1.5 minutes initially and 3 minutes at approximately 35 g Cu/liter haveresulted. At higher copper contents the etching time is furtherlengthened and becomes uneconomical. With iron chloride as etchingagent. initial etching time is about l minute, but even when ferricchloride is used as the etching agent the initially high etching rategoes steadily down with rising copper contents and, in spray apparatus,etching times become uneconomical at copper contents of about 50 gresulting in low throughput rates.

It has already" been proposed to use copper-II- chloride as etchingagent and to regenerate the copperl-chloride which reduces the etchingspeed, by means of hydrogen peroxide and hydrochloric acid. However,this type of process was not suited for industrial production since, onthe one hand, even small amounts of copper-l-chlor-ide substantiallyreduce the etching speed and, on the other hand, the addition ofhydrogen peroxide is a highly critical matter since. if too muchhydrogen peroxide is added, oxygen forms and the etching agent issubject to gassing. This again reduces the etching speed. Besides,the'gassing etching agent can no longer be pumped with adequate pressurebe cause the liquid pumps which are in particular used in connectionwith a spraying operation are not suited for moving liquid-gas mixtures.

If, on the other hand, the addition of hydrogen peroxide is too small,an adequate regenerationof the copper-I-chloride does not take place andthe residual copper l-chloride thus again contributes to a reduction ofthe etching operation speed.

It is accordingly an object of the present invention to avoid theshortcomings described and to provide for an etching process and etchingapparatus which is useful for industrial production, and in particularfor the etching of conductive plates in a clipping or sprayingoperation.

SUMMARY OF THE INVENTION This object is met by a method of etchingconductive plates of which the surface is formed by copper or a copperalloy, the said method comprising the steps of applying an etchingsolution of copper-II-chloride to the plate by immersion or spraying andregenerating the formed copper-l-chloride in the etching solution byadding hydrogen peroxide and hydrochloric acid to the solution duringcontinuing operation, the amount of the addition being controlled by acontinuous measurement of the redox potential of the etching solution byaredox electrode and the said control being adjusted to addpredetermined amounts of hydrogen peroxide and hydrochloric acidwhenever the copper-I-ion concentration reaches a predetermined value,so as to maintain a constant etching speed.

, The invention also embraces an apparatus for carrying out the methodjust defined, comprising a tank for holding the etching solution, meansfor applying the solution to the copper or copper alloy surface, a redoxelectrode immersed in said tank for measuring the redox potential, atleast one supply vessel for holding a regenerating liquid, a conduitbetween said vessel and said tank, valve means for opening and closingsaid conduit and passing liquid from said supply vessel to said tank,and control means for operatively connect ing said valve means with saidredox electrode.

The novel features which are considered as characteristic for theinvention are set forth in particular in the appended claims. Theinvention itself. however, both as to its construction and its method ofoperation, together with additional objects and advantages thereof. willbe best understood from the following description of specificembodiments when read in connection with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. la is a graph plotting the Cu ionconcentration of the etching solution against the redox potential identified as u;

FIG. lb is a partial enlarged view of the encircled portion of FIG. In;

FIG. 10 is a graph plotting the etching velocity v against the redoxpotential; and

FIG. 2 is a diagrammatic view of the apparatus of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The process of the inventionpermits to shorten the mean etching time obtainable with iron chlorideby a substantial amount, that is by about 40%. This is of course ofgreat significance for the economies of the operation.

While the initial etching speed of the process of the invention ascompared with the customary iron-III- chloride process is somewhatsmaller, this initial speed, by virtue of the process of the invention,can be maintained rather constant during the entire operationirrespective of the number of plates or sheets which pass through theetching apparatus and irrespective of the amount of copper subjected toetching.

In the apparatus of the invention, the dimensions of the conduitspassing the regenerating liquid or liquids from the supply vessel orvessels to the tank holding the etching solution should be dimensionedso as to comply with the optimum amount-ratio of the regeneratingchemicals.

ltis preferred if the valves which control the addition of regeneratingliquids are arranged to be actuated in succession so that a small excessof hydrochloric acid may thus be supplied. This will then definitelyavoid the decomposition of the hydrogen peroxide into hydrogen andoxygen.

In general, when the etching is effected by using copper-II-chloride,copper-I-chloride will be formed according to the equation Cu CuCl 2CuCl. A comparatively minute amount of copper-I-chloride. such as 4 gCu"'/l in the etching solution, which latter may for instance containl20 g Cu* /l in the form of copper-II- chloride, will noticeably slowdown the etching speed as appears from the curve in FIG. 10. However, itis of interest that the absolute value of the copper content does nothave to be particularly constant in order to obtain a high etchingspeed. Rather, it may vary between 80 g Cu/l and I30 g Cu/l. It israther important to maintain the Cu-I-chloride amount which is beingformed during the etching operation at a low value, for instance below 2g/l, and to reoxidize it as fast as possible to copper-Il-chloride.

The regeneration, from the purely stoichiometric standpoint. takes placefollowing the equation:

2 CuCl H 2 HCL' 2 CuCl 2 H O However, the actual mechanism of thereaction between these components is far more complex than expressed bythe equation and does not require discussion here. As the stoichiometricequation shows. it is necessary to add the regenerating chemicals H 0and HCl in required ratio, that is, for 34 g of H O ,2 HCl, i.e. 2 X36.5 g, must be added. For instance, if a 35% concentration H 0 solutionand a 37.5% concentration hydrochloric acid are used, the chemicals mustbe added at a ratio of 1:2, which can easily be done by means of theproper dimensioning of the conduits from the supply vessels.

The above equation of the reaction occurring during the regenerationalso shows that hydrochloric acid is used up during the regeneration.resulting in a change of the pH of the etching solution. One might atfirst think that the pH could therefore be controlled by a pH electrodewhich would adjust the amount of addition of the regenerating agent.However, in practical applications, it has been found that in thismanner optimum etching conditions cannot be obtained. If, on the otherhand, the amount of addition is controlled by a redox electrode, asproposed in the present invention, the desired object can beaccomplished with great precision.

During the etching operation there are present in the solution Cu ionsandCu ions. The potential of the electrode depends on the concentrationratio. or more properly on the activity ratio, as indicated by thefollowing formula C F according to I E,

In Cu The potential of the electrode will be positive when theconcentration of the higher oxidation stage rises and it will reach themaximum value when the Cu ion concentration is zero. With increasing Cu,ion content. the potential of the solution changes. This change will becomparatively stronger in the area of small Cu ion concentration (0 to 4g/l) than with higher increases in the Cu ion as is illustrated by thecurves in FIG. 1a and FIG. lb.

FIG. 1a illustrates qualitatively the interrelation between the etchingvelocity v and the redox potential u. The redox potential u, as will beseen, at 530 mV which occurs at a Cu* ion concentration of less than 0.4g/l (see FIG. 1b) will afford an optimally high and almost constantetching speed. It is therefore preferred to practice the invention bymaintaining this limit value during the regeneration of the etchingagent.

This change of potential is made use of in the apparatus of theinvention for controlling the addition of the regeneration chemicals.With reference to FIG. 2 it will be seen that the apparatus 1 may forinstance be used to process conductive plates 2 which consist of acopper-coated laminate. The conductive plates are passed by means of aconveyor belt 3 under the spray nozzles 4.

In order to regenerate the etching solution, the hydrogen peroxide andhydrochloric acid are fed from the supply vessels 8 and 9 through magnetvalves 10 and 11 and are furthermore pumped by pumps 12 and 13 into theetching solution in tank 5. The magnet valves 10, l l and the pumps l2,13 are controlled by the potential as determined through the redoxelectrode 14 and the control measuring device 15.

The control device comprises. in a preferred form, an amplifier, ameasuring device and a recording device which, through amplification,indicate or record the potential differences. The measuring device isprovided with two adjustable sensors which cooperate with the hand ofthe measuring device. The sensors and the hand of the measuring deviceare designed to generate a signal whenever the hand in response to achange of potential in the etching solution moves from the area of thehigher potential with a gradually increasing Cu ion concentration intothe area of a lower potential and in so moving passes the sensors andcauses a response thereby.

This response thus occurs at a definitely determined value and causesthe opening of the magnet valves 10 and 11 in succession, and likewisecauses actuation of the pumps 12 and 13 successively. Thus, theregenerating liquids H 0 and HCl are fed from the supply vessels 8 and 9into the etching solution in tank 5.

The two sensors of the control device, in order to attain a constanthigh etching speed, are adjusted to prevent exceeding a potentialdifference of about mV and thus to maintain the Cu ion content below 2g/l.

The two sensors are also spaced from each other so that the first valvewhich adjusts the hydrochloric acid flow from the vessel 9 will openearlier than the second valve which controls the H 0 flow. Conversely,when the hand moves in the other direction, the hydrochloric acid valvewill be closed at a definite time interval prior to the closing of the H0 valve. This type of adjustment will assure that there is always asmall HCl excess in the solution.

A particular advantage of the process of the invention is that theregenerating liquids can be supplied during the etching operation andthat used'up etching solution can be withdrawn likewise during theoperation. Thus, an interruption of the process. which heretofore hasalways been necessary. is avoided.

The spent etching solution also has a high re-use value since itconsists of a hydrochloric acid-containing Cu-Il-chloride solutionwithout addition of other salts, and can therefore be used in themanufacture of various kinds of copper compounds. The cumbersome andexpensive decontamination and neutralization of the drainage water.which in conventional etching apparatus is always necessary, is thusdispensed with.

EXAMPLE This example illustrates a preferred method of practicing theinvention by means of the apparatus above described.

The material treated in this example were laminates with a facing ofpure copper. The solution in the tank at the commencement of theoperation comprised 100-] 30 g copper Il-chloride per liter and aconcentration of hydrochloric acid. The solution was sprayed onto theplates at a temperature of 4050C which developed during the reaction.

After 0.4 g/l Cu -ion-concentration corresponding to a redox potentialof 530mV was reached, the Redoxelectrode by means of the control deviceopened the valve of the supply vessel holding HCl and at the same timeactuated the pump to feed HCl into the main tank. The delayed actionvalve and pump of the supply vessel holding H 0 opened shortlythereafter.

An etching speed of 35p. copper was thus reached in about 75 seconds andkept practically constant.

I claim:

1. A method for continuously etching copper conductive plates whichcomprises contacting the copper conductive plate to be etched with anetching solution of cupric chloride (copper (ll) chloride) andregenerating the etching solution containing cuprous chloride (copper(l) chloride) formed in the etching by reaction between said copperconductive plate and said etching solution by introducing hydrogenperoxide and hydrochloric acid into said etching solution, the amountsof hydrogen peroxide and hydrochloric acid introduced being regulated bycontinuously measuring the redox potential of said etching solution bymeans of a redox electrode and automatically introducing said hydrogenperoxide and hydrochloric acid whenever the cuprous ion (Cu(l)concentration in said etching solution exceeds 4 g/l in an amount atleast sufficient to reduce said cuprous ion (Cu(l) concentration belowsaid value whereby the cuprous ion concentration is continuouslymaintained below said value and a constant etching speed is maintained.

2. Method according to claim 1 wherein hydrogen peroxide andhydrochloric acid are automatically introduced into said etchingsolution whenever the cuprous ion concentration exceeds 0.4 g/l.

3. Method according to claim 1 wherein hydrogen peroxide andhydrochloric acid are automatically introduced into said etchingsolution whenever the cuprous ion concentration exceeds 2 g/l.

4. Method according to claim 1 wherein said hydrogen peroxide andhydrochloric acid are introduced in a molar ratio of 1:2 hydrogenperoxide to hydrochloric acid.

1. A METHOD FOR CONTINUOUSLY ETCHING COPPER CONDUCCTIVE PLATES WHICHCOMPRISES CONTACTING THE COPPER CONDUCTIVE PLATE TO BE ETCHED WITH ANETCHING SOLUTION OF CUPRIC CHLORIDE (COPPER (II) CHLORIDE) ANDREGENERATING THE ETCHING SOLUTION CONTAINING CUPROUS CHLORDE (COPPER (I)CHLORIDE) FORMED IN THE ETCHING BY REACTION BETWEEN SAID COPPERCONDUCTIVE PLATE AND SAID ETCHING SOLUTION BY INTRODUCING HYDROGENPEROXIDE AND HYDROCHLORIC ACID INTO SAID ETCHING SOLUTION, THE AMOUNTSOF HYDROGEN PEROXIDE AND HYDROCHLORIC ACID INTRODUCED BEING REGULATED BYCONTINUOUSLY MEASURING THE REDOX POTENTIAL OF SAID ETCHING SOLUTION BYMEANS OF A REDOX ELECTRODE AND AUTOMATICALLY INTRODUCING SAID HYDROGENPEROXIDE AND HYDROCHLORIC ACID WHENEVER THE CUPROUS ION (CU(I) )CONCENTRATION IN SAID ETCHING SOLUTION EXCEEDS 4 G/L IN AN AMOUNT ATLEAST SUFFICIENT TO REDUCE SAID CUPROUS ION (CU(I) ) CONCENTRATION BELOWSAID VALUE WHEREBY THE CUPROUS ION CONCENTRATION IS CONTINUOUSLYMAINTAINED BELOW SAID VALUE AND A CONSTANT ETCHING SPEED IS MAINTAINED.2. Method according to claim 1 wherein hydrogen peroxide andhydrochloric acid are automatically introduced into said etchingsolution whenever the cuprous ion concentration exceeds 0.4 g/l. 3.Method according to claim 1 wherein hydrogen peroxide and hydrochloricacid are automatically introduced into said etching solution wheneverthe cuprous ion concentration exceeds 2 g/l.
 4. Method according toclaim 1 wherein said hydrogen peroxide and hydrochloric acid areintroduced in a molar ratio of 1:2 hydrogen peroxide to hydrochloricacid.